In the Catalog panel (Library module), select All Synced Photographs. Select all the photos in the All Synced Photographs catalog from the grid and then drag them to the new collection (created in the previous step) in the Collections panel.

Make a note of all the collections that are currently set up for syncing with Lightroom mobile. In the Collections panel, synced collections are indicated by icon before the collection name.

Also make a note of the number of photos in the All Synced Photographs catalog.